Introduction
In this work, we present results of the investigation program SISMANTILLES I, conducted
on the central Lesser Antilles Subduction Zone where the North and South American plates
converge with the Caribbean in a roughly ENE direction, at a rate of about 2 cm/yr (DeMets
et al., 2000). In particular, to obtain precise hypocenter locations, we applied the concept of
minimum 1D model, which incorporates the iterative simultaneous inversion of velocity and
hypocenter parameters. To partly estimate the huge structural heterogeneity of the area under
study, the minimum 1D model is complemented by station corrections. After the 1D
inversion relocated hypocenters improve our knowledge on the Subduction Zone with respect
to those derived from the permanent arrays only. The vertical depth sections perpendicular to
the volcanic arc define the slab structure and the pattern of the Wadati Benioff zone.
The Minimum 1D model
Between November 1999 and January 2002, two temporary field experiments, conducted by
the Institut de Physique du Globe de Paris (IPGP), were carried out on the Lesser Antilles
Subduction Zone. The seismic network was composed of 43 three-component seismic
stations that operated from November 1999 to August 2000 and for a second period from
November 2001 to January 2002. During the second period, the landward network was
complemented by 31 three-component Ocean Bottom Seismometers (hereafter OBS). The
readings of the permanent monitoring seismometer array, primarily composed of vertical
component seismometers of the local volcano-seismological observatories of IPGP, were
added to the arrival time readings of these temporary seismometers. Since accurate
earthquake locations are of primary importance, we performed a simultaneous inversion of
velocity structure and hypocentral locations applying the concept of minimum 1D model and
using the widely known software VELEST (Kissling et al., 1994). Because uncertainties in
the hypocentre locations introduce instabilities in the inversion process, we considered only
events for which at least 10 P- and 8 S-wave arrival times were available, with a gap in
azimuth being below 180° and an rms less than 0.5 s. The resulting data-set consists of 155
well-locatable events with 4054 P- and 2617 S-onset readings. As a-priori velocity model we
used the one of Dorel, (1981). In the obtained minimum 1D model we note only small
variations with regards to the initial estimate. This occurs because a simple layered model
cannot represent huge velocity contrasts in horizontal direction. On the contrary we found
large variations in the station corrections values. For the land stations, the station-terms retrieved
by the inversion show a trend of small positive or small negative values in the Guadeloupe island
(from -0.1 s to 0.1 s), to larger negative values (to -0.4 s) in Martinique island. The OBS station-
terms are in general positive with respect to land stations indicating late arrivals, consistently
with the presence of soft mud and sediments at the sea-bottom, and they vary with their
thickness as can be derived here from multi-channel reflection seismic profiles shot over
them (Laigle et al., 2010).
Conclusions
Figure 1 shows the final hypocentre locations of the 155 seismic events obtained after the 1D
inversion. The relocated hypocenters improve our knowledge on the subduction zone because they
have smaller uncertainties with respect to the permanent array locations (Bengoubou-Valerius et al.,
2008). The vertical depth sections, perpendicular to the arc, will help define the slab structure and the
4th Saint Petersburg Conference & Exhibition 2012 Student Programme
The Future is Now
3-4 April 2012, Saint Petersburg, Russia
dip angle. For further studies, a P & S tomography local earthquake data will bring more constraints
on the structure of Lesser Antilles arc.
Figure 1 Accurate hypocenter locations (circles) as determined by the minimum 1D P and S-
wave inversion using a combined on-/offshore network. Earthquake localization obtained
with OBS contribution are marked in black. Profiles AA’, BB’ and CC’ indicate the
orientations of the cross-sections shown on the right. The dashed line with triangles (top
right corner) represents the oceanic trench. The permanent seismic stations of the IPGP
observatories and the landward temporary stations are respectively represented by white
diamonds and triangles. OBS positions are represented by squares.
References
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Mann P. [2000] GPS geodetic constraints on Caribbean-North America plate motion.
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seismological data center for the French Lesser Antilles. Seismological Research Letters, 79, 90-102.
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Kissling, E., Ellworth W.L., Eberhart-Phillips D. and Kradolfer U., [1994] Initial reference models in
local earthquake tomography. Journal of Geophysical Research, 99, 19,635−19,646.
Laigle, M., Becel, A., Kopp, H., Lebrun, J-F, and Klaeschen, D. [2010] Along-arc segmentation and
interaction of subducting ridges with the Lesser Antilles Subduction forearc crust revealed by MCS
imaging. EGU General Assembly, Vienna, Austria.
4th Saint Petersburg Conference & Exhibition 2012 Student Programme
The Future is Now
3-4 April 2012, Saint Petersburg, Russia